Congestion Control

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Unit 4 Of ACN

Unit 4 Of ACN

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  • 2. Congestion Control
    • When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestion results.
    • Because routers are receiving packets faster than they can forward them, one of two things must happen:
      • The subnet must prevent additional packets from entering the congested region until those already present can be processed.
      • The congested routers can discard queued packets to make room for those that are arriving.
  • 3. Factors that Cause Congestion
    • Packet arrival rate exceeds the outgoing link capacity.
    • Insufficient memory to store arriving packets
    • Bursty traffic
    • Slow processor
  • 4. Congestion Control vs Flow Control
    • Congestion control is a global issue – involves every router and host within the subnet
    • Flow control – scope is point-to-point; involves just sender and receiver.
  • 5. Congestion Control, cont.
    • Congestion Control is concerned with efficiently using a network at high load.
    • Techniques for detection and recovery are:
      • Warning bit
      • Choke packets
      • Load shedding
  • 6. Warning Bit
    • A special bit in the packet header is set by the router to warn the source when congestion is detected.
    • The bit is copied and piggy-backed on the ACK and sent to the sender.
    • The sender monitors the number of ACK packets it receives with the warning bit set and adjusts its transmission rate accordingly.
  • 7. Choke Packets
    • A more direct way of telling the source to slow down.
    • A choke packet is a control packet generated at a congested node and transmitted to restrict traffic flow.
    • The source, on receiving the choke packet must reduce its transmission rate by a certain percentage.
    • An example of a choke packet is the ICMP Source Quench Packet.
  • 8. Hop-by-Hop Choke Packets
    • Over long distances or at high speeds choke packets are not very effective.
    • A more efficient method is to send to choke packets hop-by-hop.
    • This requires each hop to reduce its transmission even before the choke packet arrive at the source.
  • 9. Load Shedding
    • When buffers become full, routers simply discard packets.
    • Which packet is chosen to be the victim depends on the application and on the error strategy used in the data link layer.
    • For a file transfer, for, e.g. cannot discard older packets since this will cause a gap in the received data.
    • For real-time voice or video it is probably better to
    • throw away old data and keep new packets.
    • Get the application to mark packets with discard priority.